N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) was administered orally to male Wistar rats at a concentration of 83 microgram/ml in the drinking water for 2, 4, 5, and 7 months; the rats were killed at about month 15. Intestinal metaplasia was found in the stomachs of 80-100% of the rats treated with MNNG for 4 or more months, of 37.5% treated with MNNG for 2 months, and of 10% of the controls. Metaplastic glands, composed of goblet cells and columnar cells with striated borders, were found in the pyloric region. Paneth's cells were found at the bottom of metaplastic glands in a rat treated with MNNG for 4 months. The incidence of well-differentiated adenocarcinomas of the stomach was 63-90% in rats treated with MNNG for 4 or more months and 25% in those treated with MNNG for 2 months.